It remains constant throughout an individual’s life, and is indicated by a combination of letters. Phenotype – the physical or physiological traits of an organism. Test Cross A test cross is a cross of an individual of an unknown genotype with a homozygous recessive individual, used as a method to determine the unknown genotype. This is how you determine if a particular organism is a homozygous dominant or heterozygous. Note: It is impossible to determine the genotype of an organism that is expressing he dominant trait simply by looking at their appearance.

P. 534, test cross. Incomplete Dominance Blending of the traits of two different alleles at one locus that occurs when neither allele is dominant. Co-Dominance Two alleles may be expressed equally. The situation occurs when two different alleles for a trait are both dominant. Ex. Feather color in chicken – black birds are homozygous for B, and white birds are homozygous for W. When a black rooster is crossed with a white hen, the offspring is crossed with checkered black-and-white language.

Multiple Alleles Occurs when a gene may have more than two alleles for any given trait. Ex. Blood Types. 3 Alleles for blood type. (ABA) Note: A and B are dominant over O, however A and B are co-dominant, and are equally expressed. AAA and BOO are considered Heterozygous. Human b type is only one example tot a trait that is governed by more than one These pair of alleles. Some traits are controlled by far greater numbers of alleles. Types of traits are governed by… (From textbook, p. 543)

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